1. Field of the Invention
This invention relates to a process, and an apparatus, for continuously manufacturing substrates for optical recording mediums.
It also relates to a process for preparing a stamper having fine patterns and e process for preparing a photomask.
2. Description of the Related Art
In recent years, a process making use of extrusion stamping molding as shown in FIG. 4 has been studied as a process for manufacturing substrates for optical recording mediums such as optical disks and optical cards having on their surfaces an information structure comprised of raised portions and/or engraved portions (hereinafter "preformats").
This is a process in which a molten resin sheet 401 extruded from an extruder 101 is pressed between a roll stamper 402 having on its periphery a pattern or patterns corresponding to the preformats (hereinafter "preformat pattern(s)") and a molding roll 403 provided opposingly to the roll stamper so that the preformat patterns are continuously transferred to the resin sheet. This process has attracted notice as a process that can greatly improve the mass productivity of substrates for optical recording mediums.
When the substrates for optical recording mediums are manufactured, it is sought, whatever their manufacturing process is, to settle two technical issues, i.e., to improve transfer precision of preformat patterns and to decrease birefringence (or double refraction) of substrates.
In order to further improve the transfer precision of preformat patterns to substrates and to more decrease the birefringence of substrates, it is effective to intentionally or positively control conditions for their stamping molding.
However, controlling the gap between the roll stamper and the molding roll causes a variation in thickness of substrate sheets. Such a variation in thickness of substrates for optical recording mediums further causes a non-uniformity of the quality of reproducing signals (e.g., C/N ratio) even if it is within the tolerance defined by a standard (usually, within .+-.10% of a standard thickness), and hence in recent years it has been sought to make the variation as small as possible. Accordingly, when such extrusion stamping molding is used, it has become necessary to control the variation of a gap "d" (hereinafter "gap variation") between the pattern forming surface of the roll stamper and the surface of the molding roll provided opposingly to the roll stamper to make it smaller, stated specifically, it has become necessary to make the gap variation, for example, 30 .mu.m or less, particularly 20 .mu.m or less, end more particularly 10 .mu.m or less.
Since, however, it is common for roll stampers and molding rolls to have eccentricity of their own, a great cost is required for controlling the eccentricity to produce roll stampers and molding rolls that may give the small gap variation as stated above. In an attempt to achieve the stated gap precision while using roll stampers and molding rolls having eccentricity, it becomes necessary to operate in such a very poor efficiency that a combination making the gap variation small must be selected from among a large number of roll stempars, also causing a cost increase for the production of substrates.
For the foregoing reasons, it is sought to provide a process, and an apparatus, for producing substrates with a good sheet thickness precision by extrusion stamping molding at a low cost.